(1) Field of the Invention
The present invention relates to a fuel cell and in particular to a technique for improving a current collector.
(2) Related Art
Fuel cells directly take out electric power of chemical energy of fuel, such as hydrogen, using reverse reaction of water electrolysis, which is the basic principle of fuel cells.
Fuel cells are divided into several types in terms of methods of realizing this principle. General fuel cells are alkali type, polymer electrolyte type, phosphoric acid type, fused carbonate electrolyte type, and high temperature solid electrolyte type.
The following description concerns a polymer electrolyte fuel cell, one type of the fuel cells. The polymer electrolyte fuel cell has a lamination structure which includes: a cell composed of an anode, a cathode, and a polymer electrolyte membrane therebetween; a pair of separator plates with ribs which sandwiches the cell; a pair of current collectors which are each inserted between one of the separator plates and the cell so that the current collectors contact the anode and cathode, respectively; sealing members which are each inserted between the outer regions of one separator plate and the cell. Reaction gases are supplied to the anode and cathode, respectively, to generate electric power using an oxidation reduction reaction expressed by the following formulas, namely Chemical 1 and Chemical 2. EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- (Chemical 1) EQU 2H.sup.+ +1/20.sub.2 +2e.sup.- .fwdarw.H.sub.2 O (Chemical 2)
This polymer electrolyte fuel cell is characterized by obtaining a high output, with keeping its operation temperature relatively low, compared with other type fuel cells.
The current collectors are basically required to have the following two functions other than the current collective property.
The first function is gas permeability. This function is necessary to supply and distribute reaction gases to electrode catalyzer layers which each contact one of the current collectors.
The second function is water repellence. If current collectors have high hygroscopicity, these current collectors absorb moisture of reaction gas, which is supplied after being humidified, and reaction product water during the operation of the polymer electrolyte fuel cell. As a result, the gas permeability is lowered. To prevent such situation, this water repellence property of current collectors is also necessary to realize a high-performance fuel cell.
Porous substrates, such as hydrophobic finishing carbon paper, are usually used as the current collectors to obtain current collectors having the above basic functions (see Japanese Laid-Open Patent Application No. 4-25674).
However, when current collectors of the hydrophobic finishing carbon paper are applied to a fuel cell, reaction product water and condensation product water stay between the current collectors and electrodes while the fuel cell is operating. As a result, the hydrophobic finishing affects adversely and the reaction gas is not sufficiently distributed and supplied. Note that water generated by the reduction reaction is hereinafter referred to as "reaction product water" and water generated by the condensation as "condensation product water".
In particular, when the current collectors are applied to a polymer electrolyte fuel cell, water is not sufficiently supplied to a polymer electrolyte membrane so that the polymer electrolyte membrane is dried.
The object of the present invention is to provide a current collector which solves the above problems, and to provide a fuel cell and a cell structure element including this current collector.